1. Field of the Invention
The present invention relates to a method and an apparatus for driving a plasma display panel, and more particularly, to a method and an apparatus for driving a plasma display panel with improved image quality.
2. Description of the Related Art
Plasma display panel (PDP) generally displays an image including character or graphic by exciting phosphor using ultraviolet rays with a wavelength of 147 nm, which is generated during a gas discharge of an inert mixed gas, such as He+Xe, Ne+Xe, He+Ne+Xe or the like. This PDP is easy to make slim and large-sized, and provides a greatly improved picture quality owing to the recent technology development. In particular, three-electrode alternating current (AC) surface discharge type PDP has advantages of a low voltage operation and a long life since wall charges stored on a surface in the course of discharge protect electrodes from sputtering caused by the discharge.
FIG. 1 is a view illustrating a discharge cell of a related art three-electrode alternating current (AC) surface discharge type plasma display panel.
Referring to FIG. 1, a discharge cell of the three-electrode AC surface discharge type PDP includes a scan electrode (Y) and a sustain electrode (Z) formed on an upper substrate 10, and an address electrode (X) formed on a lower substrate 18. Each of the scan electrode (Y) and the sustain electrode (Z) includes transparent electrodes 12Y and 12Z, and metal bus electrodes 13Y and 13Z. The metal bus electrodes 13Y and 13Z have line widths narrower than the transparent electrodes 12Y and 12Z, and are formed at one-sided edge regions of the transparent electrodes 12Y and 12Z.
The transparent electrodes 12Y and 12Z are generally formed of Indium-Tin-Oxide (Hereinafter, referred to as “ITO”) on the upper substrate 10. The metal bus electrodes 13Y and 13Z are generally formed of chromium (Cr) on the transparent electrodes 12Y and 12Z to reduce a voltage drop caused by the transparent electrodes 12Y and 12Z having a high resistance. An upper dielectric layer 14 and a passivation film 16 are stacked on the upper substrate 10 having the scan electrode (Y) and the sustain electrode (Z) formed in parallel with each other. The wall charge generated at the time of plasma discharge is stored in the upper dielectric layer 14. The passivation film 16 prevents the upper dielectric layer 14 from being damaged due to the sputtering generating at the time of the plasma discharge and also, enhances an emission efficiency of secondary electrons. Magnesium oxide (MgO) is generally used as the passivation film 16. A lower dielectric layer 22 and a barrier rib 24 are formed on the lower substrate 18 having the address electrode (X), and a phosphor layer 26 is coated on a surface of the lower dielectric layer 22 and the barrier rib 24. The address electrode (X) is formed in a direction of crossing with the scan electrode (Y) and the sustain electrode (Z). The barrier rib 24 is formed in parallel with the address electrode (X) to prevent the visible ray and the ultraviolet ray caused by the discharge from being leaked to an adjacent discharge cell. The phosphor layer 26 is excited by the ultraviolet ray generated due to the plasma discharge to radiate any one visible ray of red, green or blue. The inert mixed gas for the discharge such as He+Xe, Ne+Xe, He+Ne+Xe and the like is injected into a discharge space of the discharge cell provided between the upper/lower substrates 10 and 18 and the barrier rib 24.
In such a three-electrode AC surface discharge type PDP, one frame is divided into several sub-fields having different light-emitting frequencies so as to embody a gray level of the image. Each of the sub-fields is divided into a reset period in which discharges are uniformly caused, an address period in which a discharge cell is selected, and a sustain period in which the gray level is embodied according to the discharging frequencies.
For example, in case that the image is represented using a 256-gray level as in FIG. 2, a frame period (16.67 ms) corresponding to 1/60 second is divided into eight sub-fields (SF1 to SF8). Also, each of the eight sub-fields (SF1 to SF8) is again divided into a reset period, an address period and a sustain period. Herein, the reset and address periods of each sub-field are identical to each other in every sub-field, whileas the sustain period is increased in a ratio of 2n (n=0, 1, 2, 3, 4, 5, 6, 7) at each of the sub-fields. Different brightness weights of every sub-field can be combined to embody a predetermined gray level.
The conventional PDP can control the number of sustain pulses according to average picture level (hereinafter, referred to as APL) so as to uniformly deal with consumption power.
FIG. 3 is a graph showing the number of sustain pulses according to a general APL.
Referring to FIG. 3, since the brightness is determined according to the number of the sustain pulses in a PDP, if the number of all the sustain pulses of the case that average brightness is dark is identical to that of the case that average brightness is bright, problems occurs such as image quality deterioration, excessive power consumption, panel damage and the like. For example, when the number of the sustain pulses is set to be too small for all input images, contrast is reduced. In addition, when the number of the sustain pulses is set to be too large for all input images, even a dark image gets brighter and contrast is improved but the power consumption gets larger and the temperature of the panel gets higher, so that the panel may be damaged. Accordingly, it is necessary to properly adjust the number of all the sustain pulses according to the average brightness of an input image. Herein, the number of the sustain pulses increases dramatically within the gray level range in which the APL is relatively low as FIG. 3 and the number of the sustain pulses decreases within the high gray level range. Accordingly, the number of the sustain pulses increases dramatically within the gray level range in which the APL is relatively low.
FIG. 4 shows a voltage waveform in a conventional method of driving a PDP.
Referring to FIG. 4, the PDP is operated with a reset period RPD in which entire screen is initialized, an address period APD in which a cell is selected, and a sustain period in which the discharge of the selected cell is sustained.
In reset period RPD, a ramp-up signal is simultaneously applied to all the scan electrodes (Y) in a set-up duration. Small discharge is caused in cells of an entire screen to thereby generate wall charge in cells. After the ramp-up signal is applied to all the scan electrodes (Y), a ramp-down signal falling from a positive voltage lower than a peak voltage of the ramp-up signal is simultaneously applied to all the scan electrodes (Y) in a set-down duration. The ramp-down signal causes small eliminating discharge in cells thereby eliminating unnecessary charges of the wall charge and space charge generated by set-up discharge and remaining the wall charge necessary for address discharge in the cells of the entire image.
In the address period APD, a negative scan pulse (SP) is sequentially applied to scan electrodes (Y) while a positive data pulse (DP) is sequentially applied to address electrodes (X). The voltage difference between the scan pulse SP and the data pulse DP and the wall charge generated in the initialization period cause the address discharge in the cell to which the data pulse DP are applied. The wall charge is generated in the cells selected by the address discharge.
In the meanwhile, in the set-down duration and the address period APD, positive direct current (DC) voltage of the sustain voltage level Vs is applied to sustain electrodes Z.
In the sustain period SPD, sustain pulses SUSPy and SUSz are alternatively applied to the scan electrodes Y and the sustain electrodes Z. Then, as the wall charge in the cell selected by the address discharge and the sustain pulse are added, a sustain discharge is caused in the manner of surface discharge between a scan electrode (Y) and a sustain electrode (Z). Finally, after the sustain discharge is completed, eliminating ramp signal EP whose pulse width is narrow is supplied to the sustain electrode (Z) to eliminate the wall charge in the cell.
Meanwhile, in the related art, the brightness weights of the reset period RPD and the address period APD of each sub-field are identical to each other in every sub-field, whileas the brightness weight of the sustain period is increased in a ratio of 2n (n=0, 1, 2, 3, 4, 5, 6, 7) at each of the sub-fields. As described above, since sustain period SPD of each sub-field gets different, a gray level of an image can be embodied. However, since these frames are arranged identically every vertical synchronization signal as shown in FIG. 5, it is limited to represent a gray level. FIG. 5 shows the case that the number of sub-fields is 12. The number of sub-fields can be modified variously according to the gray level to be embodied.
Accordingly, in order to overcome the limitation of the gray level, it has been suggested that two frames of FIGS. 6A and 6B are alternatively arranged at every vertical synchronization signal. For example, sub-fields are arranged in odd frames (or even frames) at weight ratios of 1, 6, 13, 23, 35, 51, 70, 91, 116, 145, 176 and 211 as shown in FIG. 6A, and the sub-fields are arranged in even frames (or odd frames) at weight ratios of 4, 9, 18, 29, 42, 60, 80, 103, 130, 160, 193 and 109 as shown in FIG. 6B. As described above, if the odd frames and the even frames whose brightness weights are different from each other of each sub-field are alternatively used at every vertical synchronization signal, gray level representation ability is increased two times or more as in the case that frames whose brightness weights are identical to each other are arranged. Herein, the brightness weights of the sub-field should be alternatively allocated to the odd frames and the even frames as follows: 1, 4, 6, 9, 13, 18, 23, 29, and so on.
If the brightness weights of the sub-field are alternatively allocated to the odd frames and the even frames as described above, light emission centers are not identical to each other and flicker is caused so seriously that the, image quality deteriorated. For example, if all the sub-fields of every frame are turned on, the light emission centers of the odd frames are the position of brightness weight of 211 while the light emission centers of the even frames are the position of brightness weight of 193. Accordingly, as the locations of light emission centers of both frames are different from each other, flicker is caused to affect fatally on image quality.